The porosity and the ionic groups of the constituents of aortic tissue have been implicated in the selective retention of lipoproteins leading to atherosclerosis. However, a quantitative physicochemical study of arterial characteristics affecting the restricted diffusion and molecular sieving of solutes in this tissue has not been made so far. The objective of the proposed work will be: 1. To determine, in vitro, the permeability of rabbit thoracic aorta to diffusion of solutes of different size and charge, and to volume flow due to osmotic or hydrostatic pressure gradient; 2). From the above, to calculate the following properties of arterial wall: a. the reflection coefficient for different solutes, b. frictional interaction with different solutes, including lipoprotein, and c. distribution of pore size; 3. To determine, by electroosmosis, the ionic charge density on the arterial wall under different conditions of pH with a view to obtaining information about the nature of the ionic groups encountered by the solutes permeating through it. The experimental set-ups for all the three objectives have been completed. Preliminary results show the filtration coefficient at low pressure to be 5 times 10 to the negative 11th power cm5/dyne-sec; the reflection coefficient for inulin to be 0.26 and surface potential in physiological saline about 5 mV. By correlating the reflection and frictional coefficients with the pore size and charge of the vessel wall we hope to ascertain the importance of these factors in arterial permeability and selective retention of lipoprotein.